When it’s cold, smoke rising from city roofs is often picturesque, as in these photos from Mildred Alpern. But what exactly is that white gas? Smoke? Steam? We asked John Sideris of Sideris Plumbing & Heating for an explanation:

“What we see that emanates out of the chimney’s in many apartment buildings in NYC is the byproduct of combustion of either natural gas or #2 heating oil. To be specific, water vapor and carbon dioxide are the major byproducts of combustion. As the water vapor condenses you see a white smoke rising out of NYC chimneys. Its not smoke, just water vapor / carbon dioxide. So to answer your question, no it is not steam that we see rising out of the chimney’s in NYC of so many apartment buildings.”

if it is white and dissipates it is steam. If it is dark black (think looks like the building is on fire) it is oil. NYC is phasing out #6 and #4 oil for home heating oil or natural gas. The latter two burn ‘cleaner’ than the former 2. If it is really dark a lot of times it is a cold fired furnace. BTW the worst offender for burning #6 is the city of NYC itself…most public schools have not converted.